The present invention relates to an illuminating device for a large screen. This device makes it possible to uniformly illuminate any large surface by means of light arriving at a low angle of incidence. More particularly it makes it possible to illuminate a display screen of the liquid crystal or other type.
At present displays on large screens are produced by projection from a cell, e.g. a liquid crystal cell on to frosted glass forming a screen. The KOEHLER device for illuminating the screen is constituted by a light condenser, formed by a convergent field lens, whose surface area is equal to or exceeds that of the cell to be illuminated, and a light source positioned, for example, in the lens focus. The presently used condensers have a focal distance at least equal to the lens diameter. Therefore the depth dimension of the device is approximately the same as that of the screen, which in the case of a large screen leads to a voluminous device.